The WDM method utilizing a band characteristic of an optical fiber, is a method anticipated for enlarging the transmission capacity and constructing an optical network which is flexible in the input/output of signals. Compared to the conventional method in which light having one kind of wavelength is modulated and transmitted to one fiber at a high speed, if wavelength division multiplexing is performed using this method with the same transmission speed, the information can be increased by the number of wavelength division multiplexing and transmitted. Furthermore even if the signal is transmitted at a low speed, by wavelength division multiplexing, a similar transmission capacity can be obtained as with the conventional method for transmitting a signal of one wavelength at a high speed. With the WDM method, the interval of each wavelength to be multiplexed should be such that the wavelenghts are apart from each other so that each wavelength is not affected by the adjacent wavelength signal. At present, the band of an optical amplifier has been enlarged (to be more than ten nanometers), and a filter on the reception side having a narrow selection zone has been realized. Hence experiments of a WDM transmission system with a wavelength interval of about 1 nm have been reported, and this system is about to be introduced as an actual system.
Furthermore, the recent trend of research is to realize an optical network based on this WDM transmission system. As the optical network, as proposed in for example, Japanese Unexamined Patent Publication No. 4-167634, there can be mentioned a network having an ADM (Add-Drop Multiplexer) function such that not only WDM signals are transmitted from point to a point, but also only a light signal having a certain wavelength among wavelength division multiplexed light signals is selectively transmitted at a junction point referred to as a node provided along the transmission path, and light signals having other wavelengths are received at the node, or another light signal is inserted from the node and transmitted to other nodes. The ADM function has the characteristic in that it can freely branch or insert a signal in the optical state, and is a technique peculiar to the WDM method.
As the conventional light branching/inserting apparatus (hereinafter referred to as an ADM node apparatus), for example, as shown in FIG. 8, there is an apparatus formed by combining two Arrayed Waveguide Gratings (hereinafter referred to as AWG). The AWG is a device having a light-wave mixing function or a light-wave dividing function, and when a wavelength division multiplexed light signal is input to an input port, a light signal wave-divided for each wavelength is output from each port on the output side. On the contrary, when the AWG inputs light having a previously determined wavelength corresponding to each port, a wavelength division multiplexed light signal in which this light is wave-mixed is output on the output side. The ADM node apparatus formed using the AWG, wave-divides the multiplexed light signal for each wavelength with the AWG on a first stage (on the input side), controls the branching, insertion or transmission, respectively, for each wavelength, and wave-mixes again the light signals of each wavelength with the AWG on a second stage (on the output side) to send them to the transmission path. The control of the branching, insertion or transmission of a light signal having an optional wavelength can be realized, for example, by providing a light switch on each output port in the AWG on the first stage, and controlling the switching state.
With such a conventional ADM node apparatus, the transmission light wavelength characteristics of the AWG are designed in advance corresponding to each wavelength of a multiplexed light signal used for the transmission. Moreover, the input wavelength characteristics and the output wavelength characteristics to/from each port in the AWG are determined not optionally but relatively with periodicity. Therefore, it is important as a function of the ADM node apparatus that each port and the wavelengths of the light signals are always controlled precisely.
With the abovementioned conventional ADM node apparatus, as the number of wavelength division multiplexing increases, the control of the wavelength of light signals for each port becomes complex and the node construction becomes complicated. Moreover, in the designing stage, since the wavelength and the largest number of the wavelengths to be used in the ADM node apparatus are determined beforehand, the ADM node apparatus has a defect in that it is difficult to make this correspond to changes in the wavelengths to be used later, to addition of signals, and so on. Furthermore, the conventional ADM node apparatus has a periodic variation to the wavelength with respect to the transmission characteristics. Hence there is the problem in that with a ring network or the like to be used by connecting ADM nodes in multi stages, measures for correcting the light power for each wavelength and the like become necessary by using, for example, an optical attenuator or the like.
It is effective to use an acousto-optic filter for the ADM filter as one device for facilitating control of the wavelength of a light signal for each port. The ADM node construction using an acousto-optic filter is proposed in, for example, Japanese Unexamined Patent Publication No. 9-113855. The abovementioned technique is to suppress interference noise by shifting a frequency of branched light and insertion light, in order to prevent signal deterioration due to the interference of the branched light and the insertion light which occurs when an acousto-optic filter or the like is used for the ADM node.
With the abovementioned ADM node construction however, since complicated work is necessary for changing the branched/inserted wavelength, and for adding signals and so on, and the insertion light the frequency of which is shifted is wave-mixed with the main light signal at each ADM node and transmitted, then as the number of wavelength division multiplexing in the transmission system increases, the control of the wavelength of the light signal becomes complicated for the whole system.